Grain BaTiO

Niepce, J.C. Permittivity of fine grained BaTiOs, Electroceramics 4(Sept. 5-7), 29-39 (1994)... 

Further miniaturization of chip capacitors with thinning the dielectric layer and electrode laydown is also effective for the production cost. By incorporating thinner dielectric layers with nickel compatible dielectrics and nickel electrode, large capacitance MLCs comparable to tantalum electrolytic capacitors have been developed. To reduce the dielectric thickness less than few micrometers, further development of ultrafine grain BaTiOs is indispensable. 

FIGURE 3.1.6 Temperature dependence of dielectric constant of the ultrafine grained BaTiOs. 

Fig. 8. Dielectric constant (1 kHz) vs temperature for BaTiO ceramics of A, l-)Tm grain size, and B, 50-)Tm grain size.

The most significant commercial product is barium titanate, BaTiO, used to produce the ceramic capacitors found in almost all electronic products. As electronic circuitry has been rniniaturized, demand has increased for capacitors that can store a high amount of charge in a relatively small volume. This demand led to the development of highly efficient multilayer ceramic capacitors. In these devices, several layers of ceramic, from 25—50 ]lni in thickness, are separated by even thinner layers of electrode metal. Each layer must be dense, free of pin-holes and flaws, and ideally consist of several uniform grains of fired ceramic. Manufacturers are trying to reduce the layer thickness to 10—12 ]lni. Conventionally prepared ceramic powders cannot meet the rigorous demands of these appHcations, therefore an emphasis has been placed on production of advanced powders by hydrothermal synthesis and other methods. 

Standard ceramic processing conditions are controlled so that the resulting ceramic microstmcture is composed of semiconducting 2inc oxide grains (pZnO < IHcm) and electrically insulating grain boundaries > 10 0cm)). The microstmcture is thus similar to that of the BaTiO thermistor and... 

The PTCR effect is complex and not fully understood in terms of the grain boundary states and stmcture. Both the PTCR effect and room temperature resistivities are also highly dependent on dopant type and ionic radius. Figure 11 (32) illustrates this dependence where comparison of the PTCR behavior and resistivity are made for near optimum concentrations of La ", Nd ", and ions separately substituted into BaTiO. As seen, lowest dopant concentration and room temperature resistivity are obtained for the larger radius cation (La " ), but thePTCR effect was sharpest for the smallest radius cation (Y " ), reflecting dual site occupancy of the Y " ion. 

Single crystal and bulk BaTiOs exhibits a sharp paraelectric-to-ferroelectric transition at 393K. In the presence of submicron grains, the transition becomes diffuse and can be absent for polycrystalline BaTiOs. Twin boundaries along the four crystallographically equivalent 11 planes constitute the main lattice defects. Junctions between such twin boundaries can be frequently observed within a grain. The local atomic arrangement of the core of twin intersections was studied by focal-series reconstruction (Jia etal. 1999). 

Offer outline explanations of why the permittivity of polycrystalline BaTiOs increases with decreasing grain size. Show that the 90° domain wall area is proportional to (grain size)5/2 and that the domain wall area per unit volume is approximately 5 x 103g-1/2. Numerical data can be found in Section 5.7.1. 

The value of of the BaTiO, ceramics is lower than that reported for BaTiOj single crystal [9] (along [100] =4 000). This may be due to the structural and compositional variances. Meanwhile, the size of crystalline particle may affect the dielectric constant , that is, when the particle size is lower than certain value, the constant will decrease with the decrease of the size. In addition to those mentioned above, porosity and the existence of low dielectric constant affect the non-ferroelectric layers at the metal-ferroelectric interface and the grain boundaries. 

Tsunekawa S, Ito S, More T, Ishikawa K, Li Z-Q, Kawazoe Y (2000a) Critical size and anomalous lattice expansion in nanocrystalline BaTiOs particles. Phys Rev B 62 3065-3070 Tsunekawa S, Ishikawa K, Li Z-Q, Kawazoe Y, Kasuya A (2000b) Origin of anomalous lattice expansion in oxide nanoparticles. Phys Rev Letters 85 3440-3443 Turkovic A, Ivanda M, Popovic S, Tonejc A, Gotic M, Dubcek P, Music S (1997) Comparative Raman, XRD, HREM and SAXS studies of grain sizes in nanophase Ti02. J Molec Struct 410/411 271-273 ten Wolde PR, Frenkel D (1997) Enhancement of protein crystal nucleation by critical density fluctuations. Science 277 1975-1978... 

Figure 15.17 Effect of grain size on pemiittivity of BaTiO. ...

A co-extrusion technique was also applied by Beeaff and Ifilmas to produce a BaTiOs multilayer capacitor with nickel electrodes [Bee 02], Polyethylene co-butyl acrylate was used as the major binder component. The minor binder, plasticizer and surfactants used were polyethylene ethyl acetate, microcrystalline wax, mineral oil and PEG. With this binder system it was possible to produce multilayer capacitors with 10 im thick Ni electrode layers by co-extrusion. The limitation was given by the grain size of the ceramic powder used and not by the processing technique. 

Templated growth of materials like BaTiOs presents an attractive route to producing a textured ceramic. Templated grain growth of alumina was discussed above. The principle involved here is the use of a seed to initiate exaggerated grain growth. 

Polycrystalline lanthanum-doped BaTiOs (BLT) is one example of a PTC material. The effect is associated with GBs and is not observed in single crystals. At the GB there is a potential barrier (( )) preventing electron movement from one grain to an adjacent one (very similar to that caused by the IGF in ZnO). The GB resistance, i gb is... 

Avila HA, Ramajo LA, Reboredo MM, Castro MS, Parra R (2011) Hydrothermal s5mthesis of BaTiOs from different Ti-precursors and microstructural and electrical properties of sintered samples with submicrometric grain size. Ceram Int 37 2383-2390... 

Figure 8.9. Comparison between capillary energy lyt/r, r the average radius of grain boundary curvature) and calculated coherency strain energies stored in a (Bao.8Pho,2)Ti03 layer on BaTiOs. Specific grain boundary energy is assumed to be 0.5...

The mobility of faceted boundaries is also dependent on boundary defects. In the case of BaTi03, 111 twins induced and enhanced abnormal grain growth. On the other hand, dislocations at boundaries in SrTiOs did not enhance the boundary mobility, unlike the enhanced mobility of solid/liquid interfaces by dislocations.In this case, however, the boundary was not fully faceted ( 35% faceted). According to a recent investigation in BaTiOs, it seems that dislocations can also enhance the boundary mobility if the boundary is well faceted. 

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